In the well known helical scan video tape recorders, the magnetic video tape has a plurality of video tracks recorded in an angularly disposed direction across the tape, the individual video tracks being separated by a guard band. Each individual video track records a complete field or frame of a television picture, the vertical sync signal of the field being recorded at the top and bottom ends of each video track. A plurality of horizontal sync signals are also recorded in each individual track, for example, 262.sub.1/2 horizontal sync signals or lines per individual video track.
In the playback mode of operation, the magnetic tape passes in a helical wrap around the rotating magnetic head of the recorder, the head tracing a path centered through one of the video tracks and reproducing the television field recorded therein. As the recorder head passes off the one video track at the upper edge of the lower wrap of the tape, it passes onto the next video track at the bottom edge of the upper wrap of the video tape and reproduces the field recorded therein. Thus, the recording head traces each individual video track from bottom to top in succession. The speed of travel of the video tape and the speed of the rotating recording head are so adjusted that the recording head is made to be centered in the video track while automatically operated speed control circuits are utilized to maintain the proper tracking between video track and recording head.
When the relative speed between the magnetic head and the video tape is changed from the normal motion speed, for example where slow forward or reverse or fast motion reproduction speeds are adopted, the magnetic head no longer traces the center tracking of the video track but angles off on a different path, the actual direction being determined by the extent of the speed change as well as the direction of the change. Where the speed is decreased in the forward direction, the magnetic head will follow a path bridging over the guard band and onto the forward adjacent video track. With an increased speed, the path of the magnetic head crosses the guard band and moves onto the next rearward video track.
For each revolution of the magnetic head, one portion of the field in one track is reproduced, followed by a portion of the field from the adjacent track after the crossover. Because of the very close relationship in time between the picture recorded on one track and the picture on the adjacent track, it is difficult for the eye to detect a change in the monitor picture between the portion reproduced from one track and the following portion reproduced from the adjacent track. However, the magnetic head, in passing from one track to the other track and over the guard band therebetween, will produce a short burst of noise which appears at the junction of the two picture portions as a narrow horizontal band or series of streaks across the television picture.
Due to the fact that the magnetic head passes between the successive video tracks at a guard band crossover point which moves progressively between the bottom and top edges of the tape, for slow motion forward speeds the noise band travels from the upper edge of the picture tube down to the lower edge of the picture tube and then starts over again at the top. This noise band appearing in the picture during slow motion and continuously moving down the screen is very disturbing to the eye as the picture is being viewed. For fast motion speed and slow motion reverse, the disturbing noise band appears at the bottom of the picture and moves up.